diff --git a/fit/function.h b/fit/function.h
index e01ac17..39ab5a0 100644
--- a/fit/function.h
+++ b/fit/function.h
@@ -44,12 +44,14 @@ struct reveal_static_const_factory
     template<class F>
     constexpr reveal_adaptor<F> operator=(const F& f) const
     {
+        static_assert(is_default_constructible<F>::value, "Static functions must be default constructible");
         return reveal_adaptor<F>(f);
     }
 #else
     template<class F>
     constexpr const reveal_adaptor<F>& operator=(const F&) const
     {
+        static_assert(is_default_constructible<F>::value, "Static functions must be default constructible");
         return static_const_var<reveal_adaptor<F>>();
     }
 #endif